The invention concerns a novel method for reducing the power consumption of the stepping motor of an electronic timepiece. The invention also relates to an electronic timepiece employing the method.
In electronic timepieces which comprise a stepping motor for driving the display means, the major part of the power supplied by the electrical power source, which is generally a battery, is consumed by the motor. It is therefore important to limit the power consumption of the motor to the minimum level, in order to increase the service life of the battery or so as to be able to decrease the size of the battery, for a given service life.
In most of the present-day electronic timepieces, the motor receives drive pulses, by way of an excitation circuit, from a shaping circuit which is supplied with low-frequency signals by a frequency divider circuit associated with an oscillator which forms the time base. The duration of the pulses is fixed and is selected so as to ensure proper operation of the motor under the worst conditions, viz, low battery voltage, driving the calendar mechanism, shocks, etc. The motor therefore is over-supplied for most of the time.
The power consumption of the motor can be reduced substantially by adapting the power of the drive pulses to its instantaneous load and to its supply voltage.
One known solution comprises providing a pulse shaping circuit which is capable of producing pulses of different durations, and a device which detects rotary movement or the absence of rotary movement of the motor. The duration of the drive pulses applied to the motor is progressively reduced until the fact that a step has not been made is detected. A catch-up pulse is then applied to the motor and the power of the normal drive pulses is fixed at a higher value. That value is maintained for a certain period of time. If the motor has rotated normally during that period of time, the duration of the pulses is again reduced. Such a design does not permit permanent and rapid adaptation of the drive pulses to the motor load. Moreover, this slow mode of adaptation, and the application of catch-up pulses in the event of non-rotation, means that the power consumption is at a higher level than necessary.
Also known are circuits for controlling stepping motors, comprising means for detecting the movement of the rotor during application of the drive pulse and for interrupting that pulse when the rotor has performed its step movement or at least has rotated through a sufficient distance or has acquired a sufficient speed, to conclude that step.
For example, in U.S. Pat. No. 3,500,103, the movement of the movable member of the motor is detected by way of the voltage induced in a detection coil, and the drive pulse is interrupted when the movable member reaches either a given position or a given speed. Now, in order to effectively go on to conclude its step movement, the rotor, at the end of the drive pulse, must be in a given position and must have acquired a certain speed. Therefore, there are conditions required in respect of position, speed and motor torque, in order to interrupt the drive pulse. If only a single condition is imposed, the range of operation of the motor is restricted or, in contrast, if the condition imposed is not necessary, it is to the detriment of the level of power consumption that the control system operates.
The designs proposed in this prior patent therefore do not make it possible to optimize the duration of the drive pulses in dependence on the load and the supply voltage of the motor.
The designs proposed in U.S. Pat. No. 3,855,781, wherein the position of the rotor is detected by measuring the voltage induced in an auxiliary coil or a voltage produced by deformation of piezoelectric sensor in response to the passage of the teeth of one of the wheels of the gear train which is driven by the motor, give rise to the same disadvantages.
French Pat. No. 2,200,675 proposes detecting the variation in current in the actuating coil of the motor and interrupting the drive pulse when that current passes through a minimum value which corresponds to a maximum value in respect of the induced voltage. The limits of this detection operation are imposed by the shape of the current which depends on the time constant of the circuit, the back e.m.f. induced, and the load on the motor. In certain cases, the current minimum may disappear, which makes the control device ineffective. These disadvantages are in addition to those already mentioned hereinabove in relation to the other designs.
Moreover, U.S. Pat. No. 4,114,364 describes a circuit for controlling the duration of the drive pulses in dependence on the load of the motor, which comprises means for detecting the current in the actuating coil and means for interrupting the pulse when that current reaches a value which is equal to the ratio between the supply voltage of the coil and its d.c. resistance, that is to say, when the rotor has concluded its step movement. Also, envisaged is the possibility of interrupting the pulse before the current has reached that value. In this case also, only one positioning condition is imposed, for the control action.